Lenovo business oriented customer support (for ThinkPad owners) is generally one of the best around and is at least equal to Dell's business oriented customer support. The Dell XPS 15 is a consumer laptop so you will deal with Dell's consumer level customer support. It's not bad, but it is a step down from business oriented customer support.
The ThinkPad P50 passes the Dept of Defense's MIL-Spec requirements for operations in harsh environments. That means it is a semi-rugged laptop like most other ThinkPads that can survive rough treatment like being dropped from 5 feet onto a hard floor with no effect.
The Dell XPS 15 is has pretty good build quality, but it is not a MIL-Spec tested an is therefore not considered to be a semi-rugged laptop.
The glossy screen on the Dell can be pretty annoying depending on where you plan on using the laptop. Reflections indoors in a brightly lit room should not be too bad. But if you plan on using the laptop outdoors on a bright day, then reflections can be pretty distracting.
3.9lbs (Dell) vs 5.6lbs (Lenovo) can be pretty significant especially if you are carrying it around for a long time. Therefore, you need to decide what's more important; a matte screen or smaller/lighter laptop.
There is a solution for the Dell's glossy screen, but it is somewhat of a compromise. You can apply an antiglare screen protector like the one linked below. However due to the location of the webcam, the protector covers the webcam as well. You also must be patient when applying an antiglare screen because you do not want to have annoying air bubbles or do you want specks of dust trapped between the screen protector and the screen itself.
http/www.amazon.com/PcProfessional-Screen-Protector-Scratch-radiation/dp/B017GGF1PE
Also, antiglare screens are meant to diffuse the light that is reflected which is great if you do not like glare. However, the side effect is that it can make text and objects on the screen look just a little bit grainy. For good quality screen this should not be too bad.